diff --git a/slides/markmaker.py b/slides/markmaker.py
index aad08012..07e78062 100755
--- a/slides/markmaker.py
+++ b/slides/markmaker.py
@@ -28,7 +28,7 @@ class Interstitials(object):
 
     def next(self):
         index = self.index % len(self.images)
-        index += 1
+        self.index += 1
         return self.images[index]